There has so far been known a belt type transmission apparatus as a vehicular continuously variable transmission (hereinafter simply referred to as “CVT”). The belt type transmission apparatus comprises a drive side pulley and a driven side pulley serving as movable sheaves respectively, and an endless power transmission belt passing over the drive side pulley and the driven side pulley. Among such endless power transmission belts, there have so far been employed many push belt type CVTs each constituted by a metal ring of a stacked body having a predetermined thickness formed by stacking a plurality of belt-like thin plate metal ring and a multiplicity of elements stacked in the circumferential direction of the metal ring in a stacked state. The power transmission belt is constructed to have the multiplicity of the elements constrained in a wound shape with respect to the pulleys by the tensile strength of the metal ring, and held in pressurized contact with one another in the axial direction of the metal ring during the time period of moving from the drive side pulley to the driven side pulley, thereby making it possible to transmit drive force from the drive side pulley to the driven side pulley. The power transmission belt to be used for such a CVT is constituted by an endless band-like ring serving as a core material, and a multiplicity of elements having a predetermined thickness in the circumferential direction of the endless band-like ring and retained by the endless band-like ring. The elements are subject to being pressed toward the axial direction of the endless band-like ring, and serve to impart tensions to the endless band-like ring.
This kind of metal ring is produced by the steps of welding the ends of, for example, a maraging steel plate to form a cylindrical drum, performing a first solution treatment to make the cylindrical drum even in hardness, severing the cylindrical drum into a ring shape to make a plurality of rings each having a predetermined width, rolling the severed ring into a thin plate in the form of a thin plate metal ring, performing a second solution treatment to the thin plate metal ring to restore its shape of metal structure deformed by the rolling process, applying tension to the thin plate metal ring to correct its circumferential length to a predetermined circumferential length, and performing an aging and nitride treatment to produce a plurality of thin plate metal rings having respective circumferential lengths somewhat different from one another, and by stacking these thin plate metal rings in a nested state (see for example Patent Document 1).
Further, another method of producing the metal ring has been known with the steps of preparing a plurality of thin plate metal rings each made of maraging steel, fitting the thin plate metal ring on a circular jig larger in linear expansion coefficient than the material of the metal ring, heating the metal ring to an aging treatment temperature under the fitted state of the metal ring, and executing a circumferential length correction treatment of plastically deforming the metal ring so that the metal ring having a circumferential length small in variation with respect to the heat expansion of the circular jig can be somewhat increased in circumferential length by the jig. Through this method, the metal ring thus produced can be corrected to have a circumferential length within an allowable range even if the thin plate metal rings before being processed by the above method are somewhat varied in circumferential lengths (see for example Patent Document 2).